The prevalence of arterial disease is increasing in many countries due to the ageing of society. This trend is of particular importance for atherosclerotic vascular diseases such as coronary and peripheral vascular diseases, which are leading causes of death in the western world. In general, their treatment and therapy involves a bypass by using the autologous saphenous vein for treatment of the lower limp artery (Tyler et al.; J. Vasc. Burg.; 11:193-205; 1990) or the internal mammary artery for a coronary artery bypass (Cameron et al.; N. Eng. J. Med.; 334:216-219; 1996). One major drawback of venous grafts, however, is occlusion (stenosis), which is a consequence of systemic pressure-induced tissue degeneration, whereby one-third of vein grafts are occluded within 10 years. Furthermore, half of those show marked atherosclerotic changes (Raja et al.; Heart Lung Circ.; 13:403-409; 2004).
An increasing amount of people (up to 30% according to WHO report on cardiovascular diseases 2010) who require cardiac surgery, a vascular surgical bypass or even a dialysis shunt, cannot be provided with suitable autologous bypass material, due to pre-existing diseases or because the bypass material has already been used in previous surgery. Thus, the demand on an artificial vascular replacement material, which comprises analogous characteristics as the native counterpart, is increasing.
Beside the urgent need for small diameter grafts (as for the coronary arteries or peripheral blood vessels), there is also a considerable lack of replacement materials concerning large diameter vessels (as for a diseased aorta or for the repair of congenital cardiovascular malformations).
Existing artificial vascular prostheses have serious limitations. One major problem concerning synthetic materials used as vascular substitutes is the patency rate of the grafts due to thrombogenicity and graft occlusion.
Particularly, the tissue engineered small-diameter vascular grafts comprise several severe shortcomings (Teebken and Haverich; Graft; 5; 14; 2002), despite the development of many strategies to fabricate vascular substitutes with anti-thrombogenic properties.
Early approaches focused on surface coating of synthetic grafts by seeding endothelial cells directly onto the vascular prosthesis prior to implantation. However, these synthetic grafts still induce low-level foreign body reaction and chronic inflammation and are associated with an increased risk of microbial infections (Mertens et al.; J. Vasc. Surg.; 21:782-791; 1995).
More recent strategies focused on the creation of complete autologous, living vascular substitutes using a three-dimensional temporary vehicle seeded with autologous cells (smooth muscle cells and endothelial cells in order to line the inner lumen), which are harvested and cultivated. After proliferation in sufficient numbers, the cells are seeded onto the three-dimensional scaffolds (based on synthetic or natural material) and exposed to a physiological in vitro environment in a bioreactor system. After several weeks the tissue formation and maturation is completed and the vascular substitutes are ready for implantation. Optionally a non-scaffold based vascular tissue engineering concept via cell sheets is used. One of the main disadvantages is the time consuming preparation, which renders these artificial grafts useless for patients in need of such an artificial graft on short notice, and restricts the application to non-urgent patients.
An overview of scaffold materials used in crating grafts has been published by Schmidt and Hoerstrup. (M. Santin (ed.); Strategies in Regenerative Medicine; Chapter 7; DOI 10.1007/978-0-387-74660-9_7).
Natural scaffolds employed include, inter alia, tanned bovine carotid arteries, polyethylene terephthalat (Dacron® DuPont) meshes embedded into the collagen or a collagen biomaterial derived from the submucosa of the small intestine and type 1 bovine collagen.
Furthermore, decelluarized tissues fabricated from either vascular or non-vascular sources were applied and implanted without any in vitro cell seeding, with the assumption that they will be recelluarized by host cells in vivo. However, significant shrinkage was observed in decelluarized vessels as a result of proteoglycans being removed from the tissues during the decelluarization process. Additionally, an adverse host response, aneurysm formation, infection and thrombosis after implanting decelluarized xenografts were observed.
As permanent synthetic scaffolds, polyurethane (PU) and loosely woven, relatively elastic, polyethylene terephthalat (Dacron® DuPont) based scaffolds were applied. However, the major limitation of these materials is lack of compliance. When used for repairing or replacing smaller diameter arteries, these grafts may fail due to occlusion by thrombosis or kinking, or due to an anastomotic or neointimal hyperplasia. Furthermore, expansion and contraction mismatches can occur between the host artery and the synthetic vascular prosthesis, which may result in anastomotic rupture, stimulated exuberant cell responses as well as graft failure due to disturbed flow patterns and increased stresses.
Concerning biodegradable synthetic scaffolds, several attempts were made to apply biodegradable polymers as temporary mechanical support for in vitro generated tissues.
Particularly polyglycolic acid (PGA) or copolymers thereof, polylactid acid (PLA) and Poly-ε-caprolactone (PCL) were used as biodegradable polymers. The biodegradable synthetic material serves as a temporary scaffold and guides tissue growth and formation until the neo-tissue demonstrates sufficient mechanical properties, whereby—in theory—the scaffold will degrade completely after a certain time, providing a total autologous vascular graft. However, the difficult control of the ratio of degeneration, which has to be proportional to the tissue development, is one of the main drawbacks of these grafts. As a consequence, if the speed of material degradation is faster than regeneration of the tissue in the vascular graft, the graft may rupture.
There are many drawbacks considering the provision of artificial grafts. For example, matching the mechanical properties of large-diameter vessels for the replacement of the aorta—due to high pressure changes—is difficult. Such mechanical properties could only be obtained in long in vitro culture times, which render clinical application almost impossible. Furthermore, a long in-vitro culture time increases the risks of infection and cell dedifferentiation.
The demand for small diameter artificial grafts is very high. Especially with respect to the tissue engineering of small-diameter blood vessels, however, the mentioned problems could not be solved satisfactorily. These artificial grafts remain a particular challenge due to the lower flow velocity compared to large-diameter vessels. Bearing in mind the law of Hagen-Poiseuille, the volume of the flow is highly dependent on the radius of the tube, considering the flow characteristics of voluminal laminar stationary flows of incompressible uniform viscous liquids through cylindrical tubes with constant circular cross-sections.
The special problem associated with small-diameter grafts appears to be related primarily to the development of a fibrinous pseudointima, with gradual thickening that leads to thrombotic occlusion of the graft. However, patency rates of artificial small-diameter grafts are unacceptable in comparison to autologous vein and arterial grafts (Teebken and Haverich; Graft; 5; 14; 2002).
Thrombosis due to the reaction with foreign bodies or lack of endothelial cells, intimal hyperplasia caused by inflammatory reaction and compliance mismatch of the native vessel and the prosthetic graft at the anastomosis site are unsolved problems of particular importance.
In summary, existing grafts—especially small diameter grafts—have severe drawbacks such as the amount of time to produce in vitro grafts (e.g. via seeding of endothelial cells), thrombosis or the lack of the necessary stability.
Therefore, the provision of artificial grafts, in particular small-diameter artificial grafts, is highly desirable, in order to provide means of an optimal therapeutic artificial vascular graft, which can be used for a cardiovascular bypass operation for patients lacking suitable autologous bypass material.
It is an object of the present invention to improve on the above mentioned state of the art, in particular to provide safe and efficacious artificial grafts, which could be used instantly after unpacking, without the limitations of the existing artificial grafts, as well as a method to produce said grafts. This objective is attained by the subject matter of the independent claims.